Human clinical and epidemiological studies as well as murine disease models have clearly shown dietary fish oil to possess anti-inflammatory properties. The primary effector molecules are thought to be the n-3 polyunsaturated fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are present in relatively low quantities in the western diet. However, the immunomodulatory mechanisms of action have not been determined. We have previously demonstrated that short-term dietary exposure to highly purified EPA and DHA suppresses mitogen-induced mouse T-cell proliferation by inhibiting interleukin-2 (IL-2) secretion and IL-2 receptor alpha mRNA expression, and these events are accompanied by reductions in the production of essential lipid second messengers, diacylglycerol (DAG) and ceramide. Since incorporation of n-3 polyunsaturated fatty acids into T cell-phospholipids can alter membrane domain organization, we have hypothesized that dietary EPA and DHA suppress T-cell proliferation by altering plasma membrane lipid motional properties thereby down-modulating receptor activation following engagement. Since T-cell activation is evoked by at least two membrane receptor- dependent (two-signal model) co-activating signal transduction events and proliferation is driven by autocrine stimulation of the interleukin-2 receptor (IL-2R), we will investigate whether the T-cell receptor (TcR) (Signal 1), IL-2R, and/or CD 28- dependent signals (Signal 2) are suppressed by dietary n-3 polyunsaturated fatty acids. For these studies, mice will be fed highly purified individual polyunsaturated fatty acid ethyl esters, which allows for the simultaneous comparison of EPA and DHA without the influence of confounding molecules found in fish oil. Elucidation of the mechanism(s) by which EPA and DHA feeding suppresses T- cell function will lead to the establishment of dietary guidelines designed to reduce the incidence and severity of disorders that involve an inappropriately activated immune response (e.g., rheumatoid arthritis), without compromising host defenses.